α-Hydrazino Acids Inhibit Pyridoxal Phosphate-Dependent Decarboxylases via "Catalytically Correct" Ketoenamine Tautomers: A Special Motif for Chemical Biology and Drug Discovery?

We present evidence that supports a 'correct hydrazone tautomer/Dunathan alignment model' for how α-hydrazino analogues of α-amino acids inhibit PLP enzymes. Described is the asymmetric synthesis of l- and d-α-hydrazino acid l-lysine analogues and their inhibition of lysine decarboxylase (LdcI) via kinetic analysis, stopped-flow spectrophotometry, and cryo-EM. We describe a similar investigation of the important anti-Parkinsonism drug, carbidopa, with its human DOPA decarboxylase (hDdc) target. Evidence is consistent with these three hydrazino analogues forming the catalytically relevant ketoenamine PLP-hydrazone tautomer in their target active sites, with the α-carboxylate groups, though insulated, aligning with the PLP-π-system in a Dunathan-model-like orientation. High-resolution cryo-EM structures of the LdcI holoenzyme (pdb 9E0M-2.1Å) and LdcI-bound l- and d-hydrazones (pdb 9E0O-2.0 Å; pdb 9E0Q-2.3Å) and the first X-ray crystal structure of hDdc-bound carbidopa (pdb 9GNS-1.93Å) support this 'correct tautomer' model. These insights are expected to guide future PLP enzyme inhibitor development.